I get new messages when I launch thunderbird but I just discovered that the original menu item for get new messages was replaced with an icon. It's a stupid icon that looks like a cloud with a down arrow under it. I discovered what it was quite by accident along with the way to not show the + message button.
While I am in here let me point out another issue I don't like. I have several email addresses in use and, as a personal preference, I keep them collapsed BUT every time I re-launch thunderbird, it automatically re-expands all the folders. Why? I did not ask for this and I don't want it.